R.P 4 (Testing for ions) (+ G7 H2SO4 REACTIONS) Flashcards
What is the reagent and positive test result for sulfate ions
Write an equation for the reaction that occurs
Reagent:
BaCl2 solution acidified with hydrochloric acid
Positive test result:
A white precipitate of Barium Sulfate forms
Ba2+ (aq) + SO42-(aq) —> BaSO4(s)
Why do we need to add HCl when testing for sulfate ions
Write an equation for the reaction that occurs
The hydrochloric acid is needed to react with carbonate impurities that are often found in salts which would form a white Barium carbonate precipitate and so give a false result
2HCl + Na2CO3 —-> 2NaCl + H2O + CO2
Fizzing if carbonate ions were present
What are the 4 steps in testing for halide ions
1) Add HNO3
2) Add AgNO3
3) Add dilute NH3
4) Add concentrated NH3
Why do we add HNO3 (nitric acid) in the first step of identifying halide ions (why is silver nitrate acidified)
Write an equation for this reaction
Removes Co32- ions
(essential as Ag2CO3 is a white ppt)
2 HNO3 + Na2CO3 —> 2 NaNO3 + H2O + CO2
(Na is from compound like NaBr)
Which element is clearly identified after adding AgNO3 in the halide ion test
F-
(Gives a colourless solution)
(The others give white, cream and yellow)
Which element is clearly identified after adding DILUTE NH3 in the halide ion test. Write the equation for this reaction
Cl-
(In Cl- white ppt dissolves, Br- and I- have no visible change)
AgCl(s) + 2NH3(aq) —> [Ag(NH3)2]+(aq) + Cl- (aq)
Which element is clearly identified after adding CONCENTRATED NH3 in the halide ion test. Write the equation for this reaction
Br-
(In Br- cream ppt dissolves, I- has no visible change)
AgBr(s) + 2NH3(aq) —> [Ag(NH3)2]+ (aq) + Br - (aq)
What is the test for I- ions
No visible change after completing all 4 steps of halide ion test
Silver iodide does not react with ammonia – it is too insoluble.
What is the purpose of adding dilute NH3 in the halide ion tests
The silver halide precipitates can be treated with ammonia solution to help differentiate between them
if the colours look similar
State the colours of precipitate formed when AgNO3 is added to Cl-,Br-, and I-
Write the ionic equation for each
Chloride (Cl-) ions: Produce a white precipitate of silver chloride
Ag+(aq) + Cl-(aq) –> AgCl(s)
Bromide (Br-) ions: Produce a cream precipitate of silver bromide
Ag+(aq) + Br-(aq) –> AgBr(s)
Iodide (I-) ions: Produce a yellow precipitate of silver iodide
Ag+(aq) + I-(aq) –> AgI(s)
What is the test for the presence of carbonate ions
Write an equation for the reaction that occurs
Add dilute HCl, effervescence if carbonate ions present.
Bubble gas through limewater to test for CO2 – will turn limewater cloudy
2HCl + Na2CO3 —-> 2NaCl + H2O + CO2
What is the test for the presence of OH-
Alkaline hydroxide ions will turn red litmus paper blue
Write equations and state the observations for the reaction of NaF and NaCl with H2SO4
State what type of reaction occurs
NaF(s) + H2SO4(l) —> NaHSO4(s) + HF(g)
White steamy fumes of HF are evolved.
NaCl(s) + H2SO4(l) —-> NaHSO4(s) + HCl(g)
Observations: White steamy fumes of HCl are evolved.
F- and Cl- ions are not strong enough reducing agents to reduce the S in H2SO4. No redox reactions occur. Only acid-base reactions occur.
What is the role of H2SO4 when it reacts with NaF or NaCl
H2SO4 plays the role of an acid (proton donor).
Write equations and state the observations for the reaction of NaBr with H2SO4
State what type of reactions occur
Acid- base step:
NaBr(s) + H2SO4(l) —> NaHSO4(s) + HBr(g)
White steamy fumes of HBr are evolved.
Redox step:
2HBr + H2SO4 —> Br2(g) + SO2(g) + 2H2O(l)
Red fumes of Bromine are also evolved and a colourless, acidic gas SO2
H2SO4 plays the role of acid in the first step producing HBr and then acts as an oxidising agent in the second redox step.
Write the half equations for the reaction of NaBr with H2SO4
Ox ½ equation
2Br- —> Br2 + 2e-
Re ½ equation
H2SO4 + 2 H+ + 2 e- —> SO2 + 2H2O
LEARN HALF EQUATIONS THEN CAN PUT TOGETHER FULL EQUATIONS IN EXAM
Write equations and state the observations for the reaction of NaI with H2SO4
State what type of reactions occur
NaI(s) + H2SO4(l) —> NaHSO4(s) + HI(g)
White steamy fumes of HI are evolved.
2HI + H2SO4 —> I2(s) + SO2(g) + 2H2O(l)
Black solid and purple fumes of Iodine are also
evolved
A colourless, acidic gas SO2
6HI + H2SO4 —> 3I2 + S (s) + 4H2O (l)
A yellow solid of Sulphur
8HI + H2SO4 —> 4I2(s) + H2S(g) + 4H2O(l)
H2S (Hydrogen Sulphide), a gas with a bad egg
smell
H2SO4 plays the role of acid in the first step producing HI and then acts as an oxidising agent in the three redox steps
Write the half equations for the reactions of NaI with H2SO4
(1)
Ox ½ equation 2I - —> I2 + 2e-
Re ½ equation H2SO4 + 2 H+ + 2 e- —-> SO2 + 2H2O
(2)
Ox ½ equation 2I - —> I2 + 2e-
Re ½ equation H2SO4 + 6 H+ + 6 e- —-> S + 4H2O
(3)
Ox ½ equation 2I - —> I2 + 2e-
Re ½ equation H2SO4 + 8 H+ + 8 e- —-> H2S + 4H2O
LEARN HALF EQUATIONS THEN CAN PUT TOGETHER FULL EQUATIONS IN EXAM
Write the half equation for the formation of Br2 from Br-
State whether this is oxidation or reduction
Ox ½ equation
2Br- —> Br2 + 2e-
Write the half equation for the formation of I2 from I-
State whether this is oxidation or reduction
Ox ½ equation 2I - —> I2 + 2e-
Write the half equation for the formation of SO2 from H2SO4
State whether this is oxidation or reduction
H2SO4 + 2 H+ + 2 e- —> SO2 + 2H2O
Write the half equation for the formation of S from H2SO4
State whether this is oxidation or reduction
H2SO4 + 6 H+ + 6 e- —-> S + 4H2O
Write the half equation for the formation of H2S from H2SO4
State whether this is oxidation or reduction
H2SO4 + 8 H+ + 8 e- —-> H2S + 4H2O
DONT NEED TO LEARN BUT INTERESTING
